This invention relates to a decision-feedback equalizer for use in equalizing a demodulated signal sent from a demodulator.
In general, a demodulator is operable in response to a received signal sent through a radio channel to demodulate the received signal into a demodulated signal. The received signal may be seriously subjected to fading during transmission. The demodulated signal may be a baseband signal having a binary level or multilevel. A decision-feedback equalizer of the type described is supplied with the demodulated signal and is operated to equalize the demodulated signal into an equalized signal. As a result, the fading is moved from the equalized signal which is put into a fading-free state.
A conventional decision-feedback equalizer comprises a transversal filter having first and second filter sections. The first filter section filters the demodulated signal into a first filtered signal in accordance with first through N-th primary controllable tap gains, where N represents a positive integer. The second filter section filters an input signal into a second filtered signal in accordance with first through M-th subsidiary controllable tap gains, where M represents a positive number. The conventional decision-feedback equalizer further comprises a first producing section for producing a third filtered signal dependent upon the first and the second filtered signals, a second producing section for producing the equalized signal dependent upon the third filtered signal, and a control section for controlling the first through the N-th primary controllable tap gains and the first through the M-th subsidiary controllable tap gains dependent upon the demodulated signal and the equalized signal.
Such a conventional decision-feedback equalizer is disclosed in "DIGITAL COMMUNICATIONS" which is issued by McGRAW-HILL INTERNATIONAL BOOK COMPANY and which is written by John G. Proakis, Professor of Electrical Engineering in Northeastern University.
However, the fading inevitably and partially remains as a remaining fading in the equalized signal in the above-referred decision-feedback equalizer when the received signal is subjected to a large fading through the radio channel. As a result, the primary and the subsidiary controllable tap gains are not quickly controlled by the control section because the remaining fading remains in the equalized signal. Therefore, it is difficult for the conventional decision-feedback equalizer of the above-mentioned type to quickly equalize the demodulated signal into the equalized signal without a remaining fading.